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I didn't like that dyson spheres are built out at random, so this changes it so that the spheres are built starting from the equator where in reality the orbit would keep the initial ring in place while being built.

It shouldn't have any performance impact since this only gets called when picking a new node to work on.

Main downside at the moment is that there's a tendancy to work on the shells in order of their index when their progress is otherwise equal. I plan randomize this in a future update.